I know Jim has suggested sight-reading practice by hiding the current measure in the sheet music area so that you're forced to read and remember ahead of time. I could see a similar effect in the falling note area -- where the notes fade to invisible before reaching the keyboard keys -- that would have the same effect. I think the older DDR games had something like this.

Otherwise, why were you hoping to control the transparency? So you could see the background image/video easier?

Its not video creator, it is to allow preferred opacity of the notes. For example, by using a slider, you can make the note partially opaque (75, 50, 25), and so on to 0 or to completely opaque (100). This is really a preference, in which it is optional to the user. Setting it to 0 hides the notes automatically. In other ways, it acts like the note hide you see from the dropdown list of note colours, except it alters opacity and dragging slider to 0 percent hides the notes, and anything above 0 will show the notes as partially transparent. Look at glass for example. That has around 0, and tinted glass(black) around 50-ish. Its like what you see in many programs, you adjust alpha for something, it will fade as you play around with the values.

So one of my strongest motivations for new features comes when I am able to remove UI elements to make things less complex. And it sounds like your motivation is that you just want new UI elements to play with!